[The ruling on acting upon fingerprints]

The answer: It is permissible for the judge, in establishing financial rights and the like, to act upon fingerprints, because it has been established and become widespread that every person has his own specific fingerprint.
And at the very least, the fingerprint is a strong indicatory circumstance (qarīnah), and acting upon indicatory circumstances is recognized in the purified Sacred Law, so long as nothing stronger than it opposes it.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2